Beryllium-aluminum-magnesium composite



June 6, 1967 R. H. KROCK ETAL 3,323,880

BERYLLIUM-ALUMINUM-MAGNESIUM COMPOSITE Filed May 13, 1966 ALUMIN UM-MAGNESIUM PI-IASE DIAGRAM WEIGHT PER CENT MAGNESIUM omwv 5mm. in

o O O 0 w 0 5 0 5 5 5 5 4 4 3 ATOMIC PER CENT MAGNESIUM FIG. 2

IN VE N TORS K a. C N ONO RE w xm M MD O D T R O M T R CAW R L United States atent 3,323,880 BERYLLlUM-ALUM-MAGNESIUM COMPOSITE Richard H. Krock, Peabody, Mass, Earl I. Larsen, In-

dianapolis, Ind, and Clintford R. Jones, Arlington, Mass, assignors to P. R. Mallory & (10., 1116., Indianapolis, Ind, a corporation of Delaware Filed May 13, 1966, Ser. No. 549,993 3 Claims. (Cl. 29182.2)

ABSTRACT OF THE DISCLDSURE A ternary metal composite containing about 5085 percent, by weight, beryllium, about 13.5 to 45 percent, by weight, aluminum, and about 1.5 to 7.5 percent, by weight, magnesium.

The present invention relates to composites of beryllium-aluminum-magnesium and more particularly to means and methods for providing such composites through liquid phase sintering.

Liquid phase sintering differs from the several other types of sintering techniques in that the sintering of the compact is carried out in the presence of a liquid phase. Liquid phase sintering encompasses raising the temperature of the compressed powder metal constituents to a temperature wherein a predetermined amount of the liquid phase appears. In the liquid phase, one of the metal constituents, the solid, is progressively dissolved in the other metal constituent, the liquid. However, the quantities of these constituents are such that, at equilibrium, some solid phase always exists. It is thought that the liquid wets the solid so as to bring about favorable surface energies existing between the liquid and the solid thereby permitting solution into the liquid phase.

However, heretofore, when beryllium-aluminum-magnesium composites were developed in accordance with known liquid phase sintering techniques, it was found that the solid beryllium expelled the liquid aluminum-magnesium-beryllium alloy from the compact during liquid phase sintering. It is thought that the unfavorable surface energy equilibrium causing expulsion of the liquid is due to a tough, tenacious film of beryllium oxide which is present on each particle of beryllium.

The present invention prevents the expulsion of the liquid from the specimen by using an agency to intervene in the sintering stage. The agency either breaks down the oxide film on the beryllium or segregates to the metal oxide interface and lowers the surface energy of the liquid metal with respect to the beryllium oxide film so that the liquid metal progressively dissolves the solid metal.

The agency can be called a fluxing agent or flux, however, the agent has other characteristics which assist in wetting beryllium so as to surround the beryllium with a ductile envelope phase of an aluminum-magnesium-beryllium alloy matrix metal thereby avoiding the expulsion of the liquid from the specimen.

Beryllium has several desirable physical features which make it attractive for a variety of applications such as lightweight gears, lightweight fasteners, airplane parts or the like. However, beryllium has one major drawback which has seriously limited its commercial acceptance, that is, beryllium is inherently brittle at room temperature.

The lack of ductility of beryllium is attributed to the crystal structure of beryllium which is hexagonal close packed. During deformation, the basal planes of the hexagonal close packed structure, being the easiest to slip, are aligned along the working direction. Since slip is crystallographically difiicult perpendicular to the basal plane, the ductility of beryllium perpendicular to the primary fabrication direction is practically nonexistent.

Several tentative solutions have been advanced in an attempt to make beryllium metal sufficiently ductile so as to permit a widespread commercial acceptance of the metal. Cross-rolling and cross-forging have been suggested as fabrication methods which might enhance the ductility of beryllium. These fabrication techniques reduced the number of basal planes along the direction of rolling and resulted in improved ductility. However, the degree of improvement was far from satisfactory. The fact remained that beryllium must be classified as brittle at room temperature even utilizing the aforementioned method when ductility perpendicular to the fabrication temperature is considered. In addititon, the above mentioned technique would not be feasible where the fabrication is, by nature, solely along one axis such as swaging, drawing, and extrusion.

In recent years, attention has been directed to the fabrication of beryllium alloys not having the inherent brittleness of beryllium itself but possessing various outstanding properties of the metal such as, for example, low density combined with high strength. It is thought that US. Patent 3,082,521 fabricated the first ductile beryllium alloy by rapidly quenching the part from a temperature at which it was liquid. However, the beryllium content was not in excess of 86.3 atomic percent which is approximately 30 weight percent. Although the beryllium alloy was ductile, the density of the alloy was in excess of that of aluminum and about equal to that of titanium.

It has also been suggested that beryllium alloys might be fabricated by pressing and sintering a mix of metal powders. However, such a method results in expulsion of the matrix metal or metals from the beryllium specimen and the eventual freezing of the matrix metal or metals into globs on the surface of the solid specimen. It is thought that the expulsion of the matrix metal or metals is due to the surface energies of the solid beryllium and the various liquids formed. The unfavorable surface energy equilibrium is believed to be due to a tough, tenacious film of beryllium oxide which is present on each particle of beryllium.

A means and method have been discovered for preparing a composite of beryllium, aluminum, and magnesium containing 50 to 85 percent, by weight, of beryllium, 13.5

to 45 percent, by weight, aluminum and 1.5 to 7.5 percent, by weight, magnesium, thereby producing a composite having a density about the same as or less than that of aluminum, having high strength, and having good ductility. The ductility is due to the resulting micro-structure of the composite. By surrounding the beryllium particles with a ductile envelope phase, a composite is formed where, under load, the beryllium is so constrained by the ductile phase that it and the ductile phase deform continuously.

The 85 percent, by weight, beryllium composite showed a considerable amount of particle contiguity and would represent, it is thought, an upper limit on the percent by weight of beryllium contained by the composite. A further decrease in beryllium content below 50 percent, by weight, would raise, it is thought the density value of the composite to a value of little interest. It is thought that the ratio of aluminum to magnesium may be varied without having any substantial adverse effect on the properties of the composite.

Alkali and alkaline earth halogenide agents such as lithium fluoride-lithium chloride or the like in a determined ratio are utilized to segregate to the solid interface of the beryllium particle and either break down the film on the particle of beryllium and/ or alter the liquidsolid surface energy in the system.

aaaaeso Therefore, it is an object of the present invention to provide an agent to promote liquid phase sintering of a beryllium-aluminum-magnesium mixture.

A further object of the present invention is to provide a ductile beryllium composite having a low density'and' high strength.

Another object of the; present linventionis to provide a ductile beryllium composite having a matrix phase that ishcat treatable.

A further object of the present invention is to provide a ductile composite of beryllium in which berylliumiis the predominate ingredient.

Another object'of the present, invention is to provide a means and method of producing aductile composite of.

vide an agent; which eliminates the expulsion of a matrix metal from a beryllium specimen.

Still another object of the present invention is to provide alkali and alkaline'earth halogenide agents used in the fabrication of a beryllium composite.=

Another object of the present invention is to provide a composite of beryllium-alummum-magnesium that may be sintered to substantially theoretical density.

Yet another object of the present invention is to provide a means: and method whereby a ductile beryllium composite may be successfully fabricated in both a practical and economical manner.

A further object'of the present invention is to provide a lithium fiuoride lithium chloride agent for promoting. liquid phase sintering in a beryllium, aluminum, and magnesium mix.

Yet still another object of the present invention is to provide a lithium fluoride-lithium chloride agent wherein the constituents are used in a predetermined ratio.

The present invention, in another of its aspects, relates to novel features of the instrumentalities of the invention described herein for teaching the principal object of the invention and to the novel principles employed in the instrumentalities whether or not these features and principles may be used in the said object and/ or in the said field.

With the aforementioned objects enumerated, other objects will be apparent to those persons possessing ordinary skill in the art. Other objects will appear in the following description and appended claims. The invention re sides in the novel combination of elements and in the means and method of achieveing the combination as hereinafter described and more particularly as defined in the appended claims.

In the drawing:

FIGURE 1 is a phase diagram for binary alloys of aluminum-magnesium.

FIGURE 2 is a photomicrograph of a beryllium specimen illustrating a matrix metal expelled from the specimen by the forces of surface energy of solid beryllium and various liquids formed.

FIGURE 3 is a photomicrograph of a 70 percent, by

weight, beryllium, 27 percent, by weight, aluminum, re-' mainder magnesium composite illustrating beryllium particles surrounded by a ductile envelope phase of an aluminum-magnesium-beryllium alloy.

Generally speaking, the means and method of the present invention relate to a ductile beryllium composite fabricated by liquid phase sintering. The composite contains about 5085 percent, by weight, of beryllium, 13.545

percent, by weight, aluminum, and 1.5 to 7.5 percent, by

- weight magnesium.

' steps of mixing predetermined portions of powder beryllium and powder alloy of aluminum-magnesium or aluminum powder and magnesium powder with a predetermined portion'ofan agent selected from the group consisting of alkali and alkaline earth halogenides. The portions are pressed in a die to form a green compact. The compact is then heated to the sintering temperature. At this temperature the agent provides a favorable surface energy equilibrium between the beryllium and the aluminum-magnesium alloy so that the. alloy progressively dissolves the beryllium at'the'sintering temperature. Thereafter, the composite may be heat treated and rapidly quenched so that the heat-treating temperature structure is preserved and thealuminum is'supersaturated with magnesium.

Moreparticularly, the method of the present invention comprises'mixing powder beryllium of about 50 percent, by weight, with a powder alloy of aluminum and magnesium. An agent of lithium flouride-lithium chloride in about 0.5 to 2.0 percent, by weight, of the total metal additions is mixed with the beryllium and the alloy powder or elemental powder. The constituents of the agent are in about a one to one ratio by weight. The beryllium, the alloy powder or elemental powder, and the agent are pressed so as to form a green compact. The green compact is heated in a non-oxidizing atmosphere such as argon at a temperature of about 1000 centigrade to about 1l00 centigrade. At the aforementioned temperatures, the agent provides a favorablegsurface energy equilibrium between the beryllium and the alloy so that the aluminum-mag. nesium alloy progressively dissolves the beryllium. The microstructure of the resultant'composite consists of beryllium particles surrounded by a ductile envelope phase of I an aluminum-magnesium-beryllium alloy matrix metal. The alloy is sintered to substantially its theoretical density.

The alloy may be specially heat-treated and rapidly quenched so that the structure at the heat-treating temperature is prevented and the aluminum is supersaturated with magnesium, followed by appropriate precipitation hardening.

In carrying out the present invention, a beryllium base compact is fabricated by any suitable means such as powder metallurgy techniques. A suggested method utilizing this technique is to mix beryllium powder with an alloy of aluminum-magnesium or the elemental powders and an agent of equal parts of lithium fluoride-lithium chloride. The powders are blended and mixed by ball milling the metal powders and the flux agent. The blended and mixed powders are compacted to form a green compact by accepted metallurgical methods such as by compacting within the confines of a die or a hydraulic or an automatic press or by placing the powders in a rubber or a plastic mold and compacting in a hydrostatic press. The green compact is sintered in a non-oxidizing atmosphere such as argon or the like at a temperature of about 1000 centigrade to about 1100 centigrade. It is seen that the range of the sintering temperatures is below the 1277 centigrade melting point temperature of beryllium but above the 450 centigrade liquidus temperature of the aluminum-magnesium alloy. The aluminum-magnesium alloy will dissolve smaller beryllium particles and will dissolve the surfaces of the larger beryllium powder particles thereby surrounding the remainding beryllium particles with a ductile envelope phase of aluminum-magnesium-beryllium alloy during sintering of the compact.

The agent, lithium fluoride-lithium chloride, either breaks down the oxide film on the beryllium or segregate to the metal oxide interface lowering the surface energy of the liquid metal with respect to the beryllium oxide film. Simply, the agent causes the liquid to wet the beryllium.

Composites containing about 50 to 85 percent, by weight, of beryllium, and the remainder an alloy of aluminum-magnesium were successfully fabricated. The agent prevented the expulsion of the liquid aluminum-magnesium-beryllium alloy from the compact by the forces of surface energy, that is, prevented the formation of very fine rounded droplets of the aluminum-magnesium-beryllium alloy on the surface of the beryllium specimen. FIGURE 2 shows a beryllium specimen 20 having on the surface thereof an expelled alloy 21 of aluminum-magnesium-beryllium. Specimens from which the aluminummagnesium-beryllium alloy has been expelled have gross porosity and as a result are weak, brittle, and of little commercial value.

The composition of the agent utilized is about 50 parts, by weight, of lithium fluoride to about 50 parts, by weight, of lithium chloride. The agent provides an action, such that, upon heating or sintering of the pressed pow der mix to the temperature at which the liquid phase forms, expulsion of the melt from the specimen is elimi nated. Furthermore, it was found that solution of the beryllium into the alloy was enhanced as evidenced by the rounded particles of beryllium in the microstructure.

It was found that the amount by weight of lithium fluoride-lithium chloride agent should exceed 0.5 percent, by weight, of the total of all metal additions. It would appear that the optimum range of the agent is from about 0.5 percent to about 2.0 percent, by weight, of the total of all metal additions. It is believed that the quantity of lithium fluoride-lithium chloride agent required is related to the amount necessary to cover the total beryllium surface area. Hence, the minimum amount of agent needed would be a function of the surface area of the beryllium powder. The utilization of lithium fluoridelithium chloride agent in other than equal parts is possible. It is thought, however, that an equal parts mixture achieves optimum results.

By using the methods of the present invention and the lithium fluoride-lithium chloride agent, compacts were fabricated containing up to 85 percent, by weight, of be rylli-um, the remainder an alloy of aluminum-magnesium without the use of pressure during sintering. The composite was sintered to between about 92 and 99 percent of its theoretical density by a single sinter. The good strength and low density characteristics of the beryllium were retained and the resulting beryllium-aluminum-magnesium composite possessed good ductility.

Thus, by substantially surrounding the beryllium particles with a ductile envelope phase of an aluminum-magnesium-beryllium alloy matrix metal, the beryllium and the matrix metal deform continuously under load.

An aluminum-magnesium phase diagram is illustrated in FIGURE 1.

Magnesium is an effective material for hardening aluminum. The theory of the deformation of dispersed particle composite materials states that ductility in such a composite will be enhanced when the constrained flow stress of the matrix phase can be made as equal as possible to the flow stress of the dispersed particles. Hence, magnesium is used to harden aluminum. Once the composite has been cooled to room temperature, the effectiveness of the magnesium is brought into play by a subsequent heat treatment. In order to most effectively harden the material, the composite is heated into the complete alpha aluminum phase. It was found that heat treating the composite between 340 and 530 centigrade for about 1 to 2 hours is sufficient to completely dissolve all the magnesium in the aluminum. The composite is rapidly quenched into a satisfactory medium such as water or the like, such that the high temperature structure i preserved and the aluminum is supersaturated wit-h magnesium. Hence, the solutionizing treatment contains all the magnesium in solution. The equilibrium concentration of magnesium in aluminum at room temperature, however, is only 1.9 percent, by weight, and subsequent aging at temperatures between 250 and 325 centigrade will result in precipitation of a beta phase containing about 37%. percent, by weight, magnesium. The magnesium precipitated out of the supersaturated solid solution increases the strength of the aluminum-magnesium matrix. A distinct advantage of the beryllium-aluminum-magnesium composite is that the matrix phase is heat treatable.

Attention is directed to FIGURE 3, wherein a photomicrograph of 500 magnifications shows a composite of 30 percent, by weight, aluminum-magnesium alloy in beryllium after being etched by any suitable etching means such as a dilute solution of ammonium hydroxide and hydrogen peroxide. The areas 10 are beryllium particles. The areas 11 are the aluminum-magnesium-beryllium alloy surrounding the beryllium particles.

Example 1 shows the expulsion of the liquid from a beryllium specimen and Examples 2-8 are illustrative of the preparation of beryllium-aluminum-magnesium composites by liquid phase sintering.

Example I Expulsion of the liquid aluminum-magnesium-beryllium alloy from the solid beryllium specimen during liquid phase sintering when the agent of lithium fluoride-lithium chloride is not used in the preparation of a berylliumaluminum-magnesium composite.

A mixture of about 70 percent, by weight, of beryllium having a particle size of 200 mesh or finer was ball mill mixed with about 30 percent, by weight, of an alloy of aluminum-magnesium or the elemental powder of suitable particle size. The alloy contains percent, by weight, aluminum and 10 percent, by weight, magnesium. The milled mixture was pressed by any suitable means such as by an automatic press at a suitable pressure to provide a green compact sturdy enough to be handled. It was found that pressures of from about 15,000 to 20,000 pounds per square inch resulted in a green compact having a density from about 50 to 60 percent of theoretical density and sufficiently strong to be handled. Sintering of the compact was carried out in an argon atmosphere at about 1100 centigrade for about 1 hour. This technique, due to the surface energies of the solid beryllium and the liquid formed, resulted in the expulsion of the liquid from the specimen and its eventual freezing into rounded globs on the surface of the specimen as shown in FIGURE 2.

Example 2 A composite of about 70 percent, by weight, beryllium, 27 percent, by weight, aluminum, and the remainder magnesium.

A mixture of about 70 percent, by weight, of beryllium powder having a particle size of 200 mesh or finer was ball mill mixed with about 30 percent, by weight, of an alloy of aluminum-magnesium powder of suitable particle size. The alloy contains 90 percent, by weight, aluminum and 10 percent, by weight, magnesium. Also ball mill mixed with the beryllium and alloy powders was about 1.0 percent, by weight, of the total metal additions equal parts of an agent of lithium fluoride-lithium chloride. Mixtures of the beryllium and alloy powders were also prepared with the agent having 0.5 percent, by weight, of the total metal additions. The milled mixture was pressed by any suitable means such as by an automatic press at a suitable pressure to provide a green compact sturdy enough to be handled. It was found that pressures of from about 15,000 to 20,000 pounds per square inch resulted in a green compact having a density of from about 50 to 60 percent of theoretical density and sufiiciently strong to be handled. Sintering of the compact was carried out in an argon atmosphere at about 1000" centigrade for about 1 hour. The composite is heat-treated at about 450 centigrade for about 1 hour so as to completely dissolve all the magnesium into the aluminum. The composite is then rapidly quenched so that the heat-treating temperature structure is preserved and the aluminum is supersaturated with magnesium. The solutionizing treatment A composite of about 70 percent, by weight, beryllium, 27 percent, by weight, aluminum, and the remainder magnesium.

The procedure of Example 2 was followed using 70 percent, by weight, beryllium powder, 27 percent, by weight, aluminum powder, and the remainder magnesium powder. Individual composites were prepared using 0.5 and 2.0 percent, by weight, of the total metal additions.

Example 4 A composite of about 70 percent, by weight, beryllium, 27 percent, by weight, aluminum, and the remainder magnesium.

The procedure of Example 2 was followed using 70 percent, by weight, beryllium powder, mixed with about 30 percent, by weight, of an alloy powder of aluminummagnesiu-m. The alloy contains 90 percent, by weight, aluminum and 10 percent, by weight, magnesium. Individual composites were prepared using 0.5, 1.0, and 2.0 percent by weight of the total metal additions of the agent lithium fluoride-lithium chloride at a temperature of about 1100 centigrade using the aforementioned procedure.

Example A composite of about 50 percent, by weight, beryllium, 45 percent, by weight, aluminum, and the remainder magnesium.

The procedure of Example 2 was followed using 50 percent, by weight, beryllium powder, mixed with about 50 percent, by weight, of an alloy powder of aluminummagnesium. The alloy contains 90 percent, by weight,- aluminum and percent, by weight, magnesium. Individual composites were prepared using 0.5, 1.0 and 2.0 percent by weight of the total metal additions of the agent lithium fluoride-lithium chloride at temperatures of about 1000" and 1100 centigrade using the aforementioned procedure.

Example 6 A composite of about 60 percent, by weight, beryllium, 36 percent, by weight, aluminum, and the remainder magnesium.

The procedure of Example 2 was followed using 60 percent, by weight, beryllium powder, mixed with about 40 percent, by weight, of an alloy powder of aluminum-magnesium. The alloy contains 90 percent, by weight, aluminum and 10 percent, by Weight, magnesium. Individual composites were prepared using 0.5, 1.0 and 2.0 percent by weight of the total metal additions of the agent lithium fluoride-lithium chloride at temperatures of about 1000 and 1100" centrigrade using the aforementioned procedure.

Example 7 A composite at about 75 percent, by weight, beryllium,

8 22.5 percent, by weight, aluminum, and the remainder magnesium.

The procedure of Example 2 was followed using percent, by weight, beryllium powder, mixed with about 25 percent, by weight, of an alloy powder of aluminum-magnesium. The alloy contains 90 percent, by weight, aluminum and 10 percent, by weight, magnesium. Individual composites were prepared using 0.5, 1.0 and 2.0 percent by weight of the total metal additions of the agent lithium fluoride-lithium chloride at temperature of about 1000" and 1100 centigrade using the aforementioned procedure.

Example 8 A composite of about percent, by weight, beryllium, 13.5 percent, by weight, aluminum, and the remainder magnesium.

The procedure of Example 2 was followed using 85 percent, by weight, beryllium powder, mixed with about 15 percent, by weight, of an alloy powder of aluminummagnesium. The alloy contains percent, by Weight, aluminum and 10 percent, by weight, magnesium. Individual composites were prepared using 0.5, 1.0 and 2.0 percent by weight of the total metal additions of the agent lithium fluoride-lithium chloride at temperatures of about 1000 and 1100 centigrade using the aforementioned procedure.

The present invention is not intended to be limited to the disclosure herein, and the changes and modifications may be made in the disclosure by those skilled in the art without departing from the spirit and scope of the novel concepts of this invention. Such modifications and variations are considered to be within the purview and scope of this invention and the appended claims.

Having thus described our invention, we claim:

1. A ternary metal composite containing about 50 to 85 percent, by weight, beryllium, about 13.5 to 45 percent, by weight, aluminum, and about 1.5 to 7.5 percent, by weight, magnesium.

2. A ternary metal composite according to claim 1, wherein said beryllium is about 70 percent, by weight, said aluminum is about 27 percent, by weight, and said magnesium about 3 percent, by weight.

3. A ternary metal composite containing about 5085 percent, by weight, beryllium and the remainder an alloy of aluminum-magnesium, said alloy containing about 90 percent, by weight, aluminum and the remainder magnesium.

References Cited UNITED STATES PATENTS 1,899,631 2/1933 Norton 75150 2,193,363 3/1940 Adamoli 75150 3,082,521 3/1963 Cohen 29--497 CARL D. QUARFORTH, Primary Examiner.

L. DEWAYNE RUTLEDGE, Examiner.

R. L. GRUDZIECKI, Assistant Examiner. 

1. A TERNARY METAL COMPOSITE CONTAINING ABOUT 50 TO 85 PERCENT, BY WEIGHT, BERYLLIUM, ABOUT 13.5 TO 45 PERCENT, BY WEIGHT, ALUMINUM, AND ABOUT 1.5 TO 7.5 PERCENT, BY WEIGHT, MAGNESIUM. 